The present invention relates to novel barrier impregnants for metal refrigeration compressor castings, which prevent refrigerant gas and lubricant leakage through the casting wall, and which are resistant to contacting lubricant and/or halocarbon refrigerant, particularly to chlorodifluoromethane and dichlorodifluoromethane.
Williamitis, in U.S. Pat. No. 2,807,155, and Luck et al., in U.S. Pat. No. 3,878,112, teach lubricant-refrigerant systems for refrigeration compressors. The compressor housings are made from several ferrous castings. Generally, these cast housings cannot be leak tested until after machining and assembly. Only rarely will a faulty, porous casting, subject to refrigerant gas and lubricant leakage, be revealed during the machining process. The only recourse in the past, has been to reorder the casting from the supplier, machine it, and use it as a replacement. This, in the past, has caused a substantial time loss, and resulted in costly delays in completion and delivery of the compressor unit. Frequently, penalty costs are assessed by the customer for failure to deliver refrigeration units on time. In many instances, allowances are not given by the foundries for unsatisfactory castings.
In operation, lubricating petroleum base oils, and refrigerant solutions and gases contact the inside of the cast, ferrous housing, which contains the refrigeration compressor apparatus. These lubricating oils, especially during cold start-up, at about 20.degree. C. to 25.degree. C., contain a substantial amount of dissolved, chemically reactive halocarbon refrigerant, such as dichlorodifluoromethane gas (R12) or chlorodifluoromethane gas (R22). As a class, these refrigerants are powerful solvents for many organic polymers. The R22 is particularly reactive, having high solvency power, and is used primarily in reciprocating compressors. A large portion of the chlorofluorocarbon refrigerant dissolves in and remains in the oil until compressor operating temperatures of about 65.degree. C. are reached. As temperature increases, large amounts of the refrigerant will boil out of the oil and contact almost all the portions of the inside housing walls in the gas phase, under high pressure, i.e., over 100 psi.
Any processing of the inside porous interior of the castings, must effectively resist the chemical and solvent attack and degrading activity of the oil lubricant and/or refrigerant, and their solutions, at operating temperatures in excess of 65.degree. C. The chemical stability of any surface and porous interior treatment of the casting is critical, since the refrigeration system must be hermetically sealed at all times and reactions which denature, dissolve or destroy any of the solid material could transfer and redeposit elsewhere in the system, and thus plug up tubing, which would be fatal to the compressor operation.
Donley et al., in U.S. Pat. No. 2,961,258 teaches solid polyester resin jacket sealing units, for shafts and bearings used in pumps. These jackets are highly resistant to chemical attack from acids, alkalis and other corrosive chemicals. Hall, in U.S. Pat. No. 3,644,161, teaches radiation cured polyester resin metal coatings and films, containing monomers such as styrene, vinyl toluene, methyl methacrylate, divinyl benzene, ethyleneglycol diacrylate, tetraethyleneglycol diacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, and hydroxyethyl acrylate, among others, to aid in cross-linking. Such films, generally air inhibited, are made high energy radiation curable, to provide surface coatings.
As described in "New Methods In Impregnation of Die Castings", Impco Inc. Tech. Paper No. 2272, (presented at the 7th SDCE Die Casting Congress in 1972, by Gebhard and Scott), vacuum pressure impregnation of porous castings, with a resinous material is taught. The impregnation is followed by removal of surface resin and curing of the resin deposited in the porous casting interior, in an attempt to solve the leaking casting problem. The resins commonly used are polyester resin solutions and epoxy resins, containing no inert solvents, and producing very low or essentially no gaseous or liquid by-products on curing. The resins must meet the requirements of having good wetability and possess a low viscosity. It has been shown that these resins alone, however, are not particularly resistant to chemical and physical attack of chlorodifluoromethane gas (R22), alone or in solution petroleum oil.